Providing the general public with the ability to communicate information to specific persons at particular times is the general goal of a communication system. RF (radio frequency) communication systems provide the aforementioned capabilities in perhaps the most convenient fashion, with communication units that can be installed in a vehicle (mobile communication units) and communication units that can be easily carried in the hand or attached to the person via holster or belt-clip (portable communication units). A communication system accomplishes its purpose, in a variety of ways, by making communication resources available for use by these communication units. Communication resources may be RF frequencies or channels, or pairs of RF frequencies in the case of full-duplex communication. These communication resources may also be time slots in the case of TDM (time division multiplex) systems. Whatever form the communication resources may take, the communication system must allow communication units used by system subscribers to use these resources efficiently.
FIG. 1 illustrates, in block diagram form, a conventional analog communication system of the prior art, generally depicted by the numeral 100. A control console (101), including one or more operator positions, is shown coupled to a base station (102) over a point-to-point communication interface (103). The point-to-point interface (103) may be telephone lines, dedicated hard-wired lines, a microwave link, etc. The console (101) may also be connected to other base stations, repeaters, and satellite receivers as well-known in the art. The communication system (100) as shown supports communication among a number of mobile communication units (104) and portable communication units (105).
From time to time, it becomes necessary for the console (101) to transmit control information to the base station (102) in addition to voice, selective signalling, and other types of normal traffic information intended to be transmitted over the air. Examples of this control information include: transmitter on, transmitter off, etc. In communication systems of the prior art, it was not uncommon to use tone remote control to convey this control information. In a tone remote system, predetermined audio tones would be switched onto the interface (103), corresponding to particular control commands from the console (101). Filters in the base station (102) were responsive to these audio tones, and would cause the desired function to be activated. In other systems, d.c. (direct current) remote control would be implemented. In a d.c. remote control system, current sources of predetermined values would be switched onto the interconnecting interface (103). Devices sensitive to different current levels, such as relays with different pull-in current specifications, would then activate the desired functions. Of course, in a d.c. remote control application, the interface (103) would have to be hard-wired.
As communication systems have increased in complexity, more and more special functions have to be accommodated by any system control protocol. Simulcast, majority voting of input signals, encryption, and site selection are just a few of the functions that require support in modern systems. In the latest digital communication systems, system components are capable of being switched from analog to digital modulation.
Control protocols of the prior art do not provide a large enough command set to encompass the variety of functions available on modern systems. Prior art control protocols are relatively slow to respond, and require additional electronic components in the form of filters or relays in order to operate. In addition, prior art activation signals, such as tones or current levels, do not integrate well with voice and data information being sent to base stations and repeaters for transmission. These activation signals must be sent before voice and data, thus causing additional system delay.
Accordingly, a need arises for a data communication protocol with an extensive command set that eliminates the need for additional electronic components in order to operate. In addition, the data communication protocol should have a fast response time, and should be easy to integrate with voice and data traffic signals.